Diamond is the hardest material present on earth. Particularly, a diamond sintered body hardly suffers from chips caused by the cleavage, a shortcoming of monocrystalline diamond, and therefore is often used as a material for a cutting tool for a nonferrous material such as an aluminum-silicon alloy. Japanese Patent Publication Nos. 39-20483 and 52-12126 for example disclose sintered bodies formed by sintering diamond particles using a ferrous metallic binder such as cobalt.
Among these diamond sintered bodies, those having granules with a diamond particle size of less than 5 .mu.m or those having extra fine granules with a particle size of at most 1 m are known as highly chipping-resisting materials. Japanese Patent Publication No. 39-20483 for example discloses a diamond sintered body including fine diamond particles and ferrous metal powder which allows diamond to be dissolved and re-precipitated, and Japanese Patent Publication No. 58-32224 for example discloses a diamond sintered body including sintered diamond particles having a particle size of at most 1 .mu.m, a carbide, a nitride and a boride of a metal belonging to group 4a, 5a or 6a in the periodic table, a solid solution or mixture thereof and a ferrous alloy.
When these fine diamond particles and a ferrous metal such as cobalt or tungsten carbide-cobalt are used as starting materials for sintering, diamond particles could often grow abnormally unless the temperature and pressure conditions are strictly controlled, because such diamond particles are extremely active. Therefore, the size of the diamond particles partly becomes extremely large, which makes it difficult to provide a diamond sintered body having a particle size of at most 1 .mu.m and a homogeneous microstructure with high yields.
In order to solve this problem, there is known a method of controlling the growth of particles by providing hard particles such as tungsten carbide, cubic boron nitride and silicon carbide at diamond grain boundaries. Japanese Patent Publication No. 61-58432 for example discloses a diamond sintered body formed by adding tungsten carbide as hard particles.
This method however controls abnormal growth of diamond particles by providing hard particles having low compatibility with diamond particles between the diamond particles, thereby physically and chemically preventing direct bonding between the diamond particles, and therefore the formation of skeletons by sintering between the diamond particles is insufficient. As a result, the chipping resistance, shock resistance and thermal conductivity, i.e., essential characteristics of diamond are disadvantageously lowered.
Meanwhile, among various diamond sintered bodies, coarse grained ones having a particle size from at least 5 .mu.m to at most 100 .mu.m are generally known as having high wear resistance. Such coarse grained diamond particles are, however, not easily sintered, and therefore a known method forms a carbide on the surface of diamond particles in order to make sintering easier. Japanese Patent Laying-Open No. 63-134565 for example discloses a method of producing a carbide on the surface of diamond particles, thereby enhancing the binding force of a sintering aid metal to individual diamond particles to ease sintering. Many products of diamond sintered body using sintering aid including a tungsten carbide in order to ease sintering are manufactured.
When a carbide is thus generated on the surface of diamond particles, however, the wear resistance, chipping resistance, shock resistance and thermal conductivity are lower than diamond sintered bodies including only diamond particles and a ferrous metal. If a tungsten carbide is added to a sintering aid, the content of the sintering aid increases, which is more likely to deteriorate the wear resistance of the diamond sintered body.
In recent years, harder materials difficult to cut have increased the need for machining tools with a diamond sintered body for cutting these harder materials. As a result, it is required that the sintered body has a wear resistance, a chipping resistance, a shock resistance and a thermal conductivity higher than conventional sintered bodies for making cutting tools.
The present invention is directed to a solution to the above-described problems, and it is an object of the present invention to provide a diamond sintered body having a required wear resistance, chipping resistance, shock resistance and thermal conductivity higher than conventional diamond sintered bodies.